1. Field of the Invention
This invention relates generally to wellbore drilling systems and other downhole devices that utilize fiber optics.
2. Description of the Related Art
The oilfield industry currently uses two extremes of communication within wellbores. The classification of these two extremes relate to the timing of the wellbore construction. On extreme may occur during the wellbore construction whereas the other extreme may occur after wellbore construction and during the production of hydrocarbons. During the drilling and completion phases, communication is accomplished using a form of mud pulse telemetry commonly utilized within measurement while drilling (MWD) systems. Alternative methods of telemetry, such as low frequency electromagnetic and acoustics, have been investigated and found to be of limited or specialized use. In general MWD telemetry is bound by the speed of sound and the viscous properties in the drilling fluid. Thus, data rates for mud pulse telemetry seldom exceed 10 bits per second.
An increase in the number and complexity of downhole sensors in MWD systems has increased the need for higher data rates for the telemetry link. Also, the introduction of rotary closed loop steering systems has increased the need for bi-directional telemetry from the top to the bottom of the well.
Industry efforts to develop high data rate telemetry have included methods to incorporate fiber optic or wire technology into the drillstring, transmitting acoustic signals through the drill string, and transmitting electromagnetic signals through the earth surrounding the drill string. U.S. Pat. No. 4,095,865 to Denison, et al, describes sections of drill pipe, pre-wired with an electrical conductor, however each section of pipe is specially fabricated and difficult and expensive to maintain. Acoustic systems suffer from attenuation and filtering effects caused by reflections at each drill joint connection. Attempts have been made to predict the filtering effects, such as that described in U.S. Pat. No. 5,477,505 to Drumheller. In most such techniques, signal boosters or repeaters are required on the order of every 1000 feet. Thus, to date, the only practical and commercial method of MWD telemetry is modulation of mud flow and pressure, which has a relatively slow data rate.
Once a well is drilled and completed, special sensors and control devices are commonly installed to assist in operation of the well. These devices historically have been individually controlled or monitored by dedicated lines. These controls were initially hydraulically operated valves (e.g., subsurface safety valves) or were sliding sleeves operated by shifting tools physically run in on a special wireline to shift the sleeve, as needed.
The next evolution in downhole sensing and control was moving from hydraulic to electric cabling permanently mounted in the wellbore and communicating back to surface control and reporting units. Initially, these control lines provided both power and data/command between downhole and the surface. With advances in sensor technology, the ability to multiplex along wires now allows multiple sensors to be used along a single wire path. The industry has begun to use fiber optic transmission lines in place of traditional electric wire for data communication.
While conventional system utilizing fiber optics provide some additional functionality versus prior wellbore communication and measurement systems, advances in wellbore drilling technologies have to date outpaced the benefits provided by such conventional fiber optic arrangements. The present invention is directed to addressing one or more of the above stated drawbacks of conventional fiber optic systems used in wellbore applications.